Syrian rebels regrouped, seized on weakness of government’s key allies  

A lightning advance by insurgents over the past two days in parts of northern and central Syria was the result of better organization by rebel forces, the weakening of President Bashar al-Assad’s military allies and, possibly, luck: Few expected the Syrian army to collapse as quickly as it did.

On Sunday, opposition forces were in control of much of Aleppo, the northern city where Assad’s troops and allied militias vanquished rebel fighters eight years earlier in what seemed then to be a turning point in the country’s bitter civil war. Video footage suggested government troops had either retreated or melted away, allowing a long-planned offensive to advance further than expected, experts said.